Dengue virus (DENV) is a member of the Flaviviridae family of viruses characterized by a single stranded RNA genome enclosed within a spherical enveloped virion. Four distinct serotypes of DENV exist (DENV1-4; 65% conservation), each capable of causing disease following transmission by the arthropod vectors Aedes aegypti or Aedes albopictus. Although the large majority of primary infections are asymptomatic, DENV infections can cause a flu-like illness characterized by fever, fatigue, muscle pains, and rash and, in a small proportion of infected individuals, a severe form of disease called dengue hemorrhagic fever. Immunological memory protects individuals from reinfection with the same but not distinct serotypes and the more severe forms of disease most often occur during secondary infection with a different serotype. Current estimates indicate between 100 and 300 [6] million dengue virus infections occur throughout the world each year and people in over 100 countries are currently at risk of being infected with DENV. Therefore, dengue virus is now considered a “priority pathogen” by multiple governing bodies.
CD8+ T cells (CTLs) are a major contributor of protection against dengue virus infection. DENV specific CD8+ T cells have been detected in patients after natural infection and attempts at vaccination with some level of cross-reactivity between strains. Studies in children indicated that CD8+ T mediated secretion of IFN-gamma and TNF-alpha was more robust in children with asymptomatic, or subclinical, infections compared to those with symptomatic or severe disease. Likewise, in murine models of dengue virus infection, CD8+ T cells expanded robustly, secreted pro-inflammatory cytokines, possessed cytotoxic capabilities and their depletion prior to infection significantly altered the course of infection. Together, these studies indicate that successful clearance of and protection from secondary dengue virus infections requires a multifunctional and cross-reactive CD8+ T cell response. Additionally, CD8+ T cells directed against each of the viral proteins have been detected indicating that broad dengue specific T cell responses may also be necessary for protection.
To date, no specific dengue virus treatments or vaccines have been approved for use in infected or at risk individuals. Multiple vaccine strategies have been attempted and a number are in various stages of development including live attenuated viruses, inactivated viruses, and peptide based vaccines but most have achieved limited success. Interestingly, live-attenuated vaccine strategies induce the most robust B and T cell responses after vaccination indicating that efficient protection may be achieved through activation of B and T cells through alternative means. Therefore, determining the peptide epitopes that are naturally generated by antigen presenting cells during a dengue virus infection would allow for the development of other vaccine formulations (i.e. peptide based) that can induce robust and cross reactive T cell responses. The major caveat of a peptide identification approach, however, is the variability in HLA molecules that exists worldwide. The large majority of dengue infections occur in dengue endemic regions (i.e. Brazil, Asia) where the HLA-A24 molecule is most common (http://www.allelefrequencies.net/). However, with the recent emergence of dengue virus infections in other regions of the world, successful vaccine formulations must include peptides capable of binding multiple HLA alleles. For example, peptides restricted to the HLA-A2 allele and peptides restricted to the HLA-A24 allele must be included in the same vaccine formulation. Interestingly, peptides generated by the processing machinery are capable of binding to multiple HLA class I molecules; this property has allowed HLA molecules to be categorized into supertypes. Peptides that bind to one member of the MHC superfamily are likely to bind to other members of the same family. This property makes it possible to identify single peptides that bind to and induce T cell activation in multiple different MHC class I settings.